Conventional general-purpose fluororubber has very excellent resistance as oil sealing materials for engine oil, fuel oil, etc., but is not suitable as water sealing materials for cooling water, LLC (long life coolant), etc. For sealing materials for cooling devices, such as cylinder liners, oil coolers, and radiators, silicone rubber, EPDM, or a specific grade of fluororubber is generally used.
However, silicone rubber has problems, such as lack of hardness and inferior hydrolysis resistance. Moreover, as for EPDM, degradation of the rubber may occur because of hypochlorous acid derived from chlorine, which is generally added to tap water. This causes black water phenomena, in which black foreign substances are mixed into water, and peripheral equipment is contaminated. Furthermore, tap water contains many ionic substances, other than hypochlorous acid, and such various ions lead to degradation of the rubber when the rubber is used for application in direct contact with tap water. In addition, similar problems occur when EPDM is used for transport and storage of solutions containing these various ions.
A specific grade of fluororubber is superior in hot water resistance and steam resistance compared to general-purpose fluororubber, but is problematically inferior in low-temperature characteristics and durability (indicated by the rate of change of compression set values). Moreover, perfluoropolyether type fluororubber (e.g., Kalrez, produced by DuPont·Dow Elastomers) has extremely excellent heat resistance, but has very poor compression set characteristics. Furthermore, their price is very expensive. Accordingly, such fluororubber is less suitable for molding materials for general-purpose sealing materials.
In contrast, products obtained by polyol vulcanization of binary type fluororubber (e.g., vinylidene fluoride-hexafluoropropene binary copolymer rubber) are superior in compression set characteristics but inferior in chemical stability. Thus, they have possible defects, such as cleavage of crosslinking sites caused by hydrolysis, against hot water and steam. Moreover, products obtained by polyol vulcanization of ternary type fluororubber (e.g., tetrafluoroethylene-vinylidene fluoride-hexafluoropropene ternary copolymer rubber) have higher fuel oil resistance than binary type fluororubber, but tends to have lower compression set characteristics. Furthermore, as for products obtained by amine vulcanization, the rubber is eroded by amine contained in fuel oil, sometimes causing curing defects, which are generally called “amine cracks”.
The present applicant has previously proposed a method for producing fluororubber vulcanization-molded products suitably used as grommets, seal packing, etc., for sensors operable in high-temperature environments, using general-purpose fluororubber, wherein a fluororubber composition comprising a fluororubber, calcium hydroxide, magnesium oxide, and thermal black and a bituminous coal filler is subjected to vulcanization-molding in the presence of a polyol-based vulcanizing agent, followed by heat treatment at a temperature of about 250 to 300° C.
Fluororubber vulcanization molded products obtained by this method have excellent compression set characteristics and compression cracking resistance in high-temperature environments; however, they are polyol-vulcanizates and are therefore not in a satisfactory level in terms of water resistance and steam resistance (Patent Document 1).